Apparatus for distilling mineral oils



Oct. 8, 1940. 1 E, SCHULZE 2,217,386

APPARATUS FOR DISTILLING MINERAL oILs Original Filed May 5, 1932 2Sheets-Sheet 1 T0 K4 CUI/M PUMP if f/ ,LA7 ATTORNEY CCL 8, 1940s .J.' E.scHuLzE APPARATUS FOR DISTILLING MINERAL OILS Original Filed May 5. 19322 Sheets--Shee*t 2 s lNVENTOR :2l- MA() i@ ATTORNEY Patented Oct. 8,.1.940l

UNITED STATES l PATENT i 'orrlcl'vv APPARATUS ron ms'mmo MINERAL onsJohn E. Schulze, Chicago, Ill., assig'nor, hy mesne assignments,l toHigh Vacuum Processes, Inc., New York, N. Y., a corporation of DelawareOriginal application May 5, 1932, Serial No. 609,508. Divided and thisapplication June 4,

1937, Serial Nov 146,494

40mm. :(01. 19e- 114) A ployed, as well as from nearly all of thelighter I type for the manufacture of mineral lubricating oils asoverhead distillates under low absolute pressure and more economicallythan has heretofore been possible, of stable lubricating oil distillatesrequiring no acid treatment or filtration to render them marketable asfinished high grade lubricating oils of good color and stability.

A further object of the invention is to accomplish this objective byproviding a novel apparatus system in which not only the generalcombination but also the component apparatus units shall possess novelfeatures of construction and operation.

With this general object in view, as well as others which will becomeapparent hereinafter, the invention comprises the novel apparatus partsand combinations thereof which will first be described in connectionwith a typical illustrative embodiment within the scope of theinvention, and will then4 be more particularly pointed out in theclaims.

Although the novel apparatus system may be employed to advantage invarious connections, it is particularly useful in separating lubricatingoil components from mineral oil charging stock by flash-vaporizationunder low absolute pressure, as described in Patent No. 2,088,616aforesaid. As. therein set forth, a relatively quick separation oflubricating oil components from the heavier components of a given crudemineral oil material Aor charging stock is eected by a continuous'procedure which includes rapidly heating the crude material totemperatures sufliciently high to vaporize all the desired lubricatingoil components under absolute pressure on the order of 50 millimeters ofmercury or lower, vaporizing such components at such low absolutepressure, and condensing them under such low pressure as one or morelubricating oil distillates substantially free of tarry and otherheavier constitutents of the crude starting material emconstituentsthereof.

By proceeding in the manner abovedescribed,

a number of highly important advantages are realized. `Separation of theheavier and more heat-sensitiveconstituents of the crude charging stockfrom the lubricating oil constituents being accomplished quickly, theobjectionable long continued heating of the heavier constituentsnecessarily involved in batch distillation of the crude charging stockis avoided. It is true that l such quick separation requires heating thecrude charging stock to temperatures as high as, say, 725 F., or'higherthan can be employed under any conditions without some breaking down orcracking of the heavier oil components, including some of the heavylubricating oil components; but the time factor is sogreatly reducedthat the loss of lubricants from cracking is relal tively slight.

terial may be heated to the necessary tempera-` ture and passed througha series of flash vaporizers to Ydistil off Whatever gas oil or othernonviscous fractions the material may contain, and also to distil offthe desired lubricating oil fractions fromthe lightest and least Viscousto the heaviest and most viscousrleaving unvaporized, as flux oil (e.g), a tarry residue of heavier components of the crude charging stock.The vapors produced in each vaporizer may be passed upwardly through afractionating tower or column constructed and adapted to minimize vacuumdrop through the tower (increase in absolute pressure from the toptoward the bottom) as much as possible. Whatever liquid residualmaterial remains unvaporized in any given flash vaporizer is passed intothe next succeeding vaporizer for re-flashing, ordinarily after beingheated to a higher temperature, to vaporize such further lubricatingcomponents as can be thus obtained therefrom.

An important specific featureof the invention, essential for minimizingvacuum drop as aforesaid, resides in the employment of fractionatingmeans functioning effectively and yet with relatively very smallresistance to passage of the vapors which, under the conditionscharacterizing the high vacuum distillation here involved, travel atvery high velocity. In particular, it is desirable to give the oilvapors a whirling movelso nient in contact with counterilowing reflux ina vention is not in any sense restricted to the i conspecific detailshere illustrated vbygyvay of a crete explanatory example.

The accompanying drawings illustrate more or. less diagrammatically, andto some extent in the form of a ilow sheet, a typical distillation plantembodying this invention. In-these drawings,..VL

Fig. 1 illustrates the primary distillation unit of the system, showingthe .principal apparatus or equipment parts in side elevation, partsbeing broken away and in section to disclosev certain structuraldetails; and

Figs. 2, 2, 3 and 3 are views of certain parts in greater detail.

The distillation unit or system (Fig. 1), in the specic form hereillustrated, is adapted and arranged for continuous distillation ofmineral o`il charging stock by a succession of iiash vaporizationswhereby all the. lubricating oil components of the charging stock, fromvery light to extra heavy, together with anyligher fractions,

vsuch as gas oil, which the charging stock may contain, are continuouslyvaporized at low absolute pressures, the resultant vapors beingfractionated to separate the lubricating values in several cuts orfractions, as may be desired, and also to obtain the gas oil mainly asanother cut or cuts separate from the lube cuts, while rapidlywithdrawing from the various condensed lube distillates uncondensedvapors and gases including a substantial proportion of whatevermalodorous and other contaminating matters may be present. Some of thesecontaminants may be condensed with one or more gas oil cuts.

5 As here illustrated, the distillation unit comprises towers I0, II andI2, Ytogether with pipe still I3 cooperating with tower II, and pipestill I4 cooperating with tower I2. In the present example, theapparatus system illustrated is adapted to handle'crude charging stock,such as a topped crude petroleum containing little or no material morevolatile than gas oil. The distillation and quick separation oflubricating con- .stitutents proper from such charging stock is precededby an initial hashing-off in tower Il) of some of the gas oil and suchsmall amount of lighter constituents as may be present in the vchargingstock. The heat for this initial flashing oir may be obtained throughrecovery of heat from a later stage of the fiash-vaporizing procedurecarried out in a succeeding tower. 'I'he crude charging stock is drawnthrough supply line I5 by pump I8, and after being sent through line I1to certain heat exchange means (61, 13, 98) and being heated to around375400 F., as will hereinafter appear, it is returned through lline I8and is discharged by the pump into tower I0 as indicated at I8. Wherethe crude charging Y stock contains salt water, as is oiten the case,

this should be removed by passing the stock through a dehydratorinterposed at a point in line I1 where the stock has attained atemperature o f, say, 300 F. In return line I8 is interposed a mixing'coil 2I where a suitable .quantity of caustic soda .solution may bethoroughly mixed with a flowing stream of preheated charging stock-thecaustic soda solution being introducedinto the line through pipe 22which enters the line in advance of the mixing coil 2l. The preheatedcharging stock is delivered tangentially into tower I0 at a pointslightly above the stripping plates 23 contained in the lower' part ofthe tower. Just above the point of charging stock delivery orinlet is anentrainment separator 24 so designed as to impart considerable velocityand centrifugal action to the vapor ascending the towerfwith a minimumdrop in vacuum, to throw out any liquid particles entrained with thevapor, thus permitting a clean separation of such particles from thevapor prior to entering the fractionating zone located above theentrainment separator.. The construction of the entrainment separator isshown'in greater detail in Figs. 2 and 2B. The vapors rising to plate24| are deiiected laterally by curved vanes 242 which direct the vaporsagainst screen 243 while the vapors are also given a whirling movement.Entrained liquid particles arrested by the screen drain downwardly alongthe walls of the Column.

ating with downwardly sloping side rings 26.

'I'he lowest of these plates has a cooperating side ring 21 which slopesupwardly, the combination constituting a collecting plate. Curved guidevanes 21| (Figs. 3 and 3a) serve to impart whirling movement to theascending vapors. The higher boiling portions of the ascending vapors,which consist mainly of gas oil, are condensed in reux condenser 28, theliquid reilux condensate being discharged through distributor head 28|at the top of the column and flowing down overthe fractionating plates,of which there are eight in the present example, in intimate contactwith the ascending vapor. plates are so designed as to give intimatecontact of counterilowing reflux and vapor they are of such character asto cause minimum vacuum drop in the column. This type of plate or trayhas been found to be particularly suitable and to be far superior forthis purpose to bubble plates, the use of which has heretofore beenattempted in lubricating oil distillation but has been far fromsatisfactory. Other specific designs of plate can of course be employedWithin the broad scope of the invention.

Gases, lighter vapors and water vapor escaping condensation in the reuxcondenser 28 pass to a iinal condenser 29 where a still lighter gas oilis condensed together with some malodorous and unstabilizingconstituents. The remaining uncondensed vapors and gases are swiftlysucked away through large-diameter oitake 30 by suitable exhaustingmeans, such as a 2-'stage vacuum pump of the ejector type, convenientlyindicated at 30|. The exhausting means is so operated as to maintain theentire interior of tower I0 under low absolute pressure. This absolutepressure should not exceed millimeters of mercury as While the a maximumat any point in the colunm, and it is desirable in practice that thisvmax1mum should be substantially less than 50 millimeters, say 30millimeters. Due to the resistance offered to the .upward flow of vaporsby the entrainment separator and fractionating plates, the absolutepressure at the point of initial vapor1zation,' that is,

Vat the point where the charging stock is delivered into the column,will be somewhat higher than the pressure at the top of the column.Where amenace plates of the character here illustrated are employed,however, this'vacuum drop or pressure increase is reduced to a minimumand considerably below what is possible with a bubble-tray system ofcomparable Iractionating enect. Thus, assuming that the absolutepressure at the point of vaporization is to be maintainedat'substantially 30 millimeters, this can be accomplished by operatingthe vacuum pump to maintain an absolute pressure of 25 millimeters atthe top of tower I0 just before the vapors enter tne reflux condenser28.

The light gasoil and malodorous fraction or cut condensed in the finalcondenser 23 is led away through line 3| to constant level seal tank 32,from which it is withdrawn b'y pump,33 and delivered through line 34 tostorage. The seal tank has a pressure balancing connection 32| to vacuumline 30. Pump 33 is located a suillclent distance below the seal tank togive the necessary suction head, and the rate of operation of the pumpis automatically controlled in a wellknown manner by appropriateconnections 'to iioat 34| in the seal tank to maintain the liquid'levelin the seal tank constant.

The higher boiling gas oil fraction caught by collecting plate 21 istaken from the tower through line 35 to seal tank 36, from which it iswithdrawn by pump 31 and pumped through cooler 38 and thence throughline 39 to storage, the operation of the pump 31 being automaticallycontrolled, by means similar to that 'already described fr pump 33, tomaintain a constant level of the liquid distillate collecting in sealtank 35. The seal tank is provided withI pressure balancing connection36| to a point in the column at least one plate higher than where line35 leaves it or, of course, this balancing connection may go to thevacuum line (e. g. 30). K

That portion of the charging stock which does not vaporize upon deliveryinto tower I0 passesv down over stripping plates 23 and accumulates inthe base of the tower to the level indicated at 40, this level beingmaintained constant by float control mechanism 4| which controls therate of operation of charging stock feed pump I6 through regulation ofits steam supply, the arrangement being the same as described by pump 33and seal tank 32. y This unvaporized residue of liquid oil containingsubstantially all the original lubricating oil constituents of the rawcharging stock, together with most of the gas oil, is next heated to amuch higher temperature in order that a large part of the lubricatingconstituents may be dash-vaporized therefrom in tower at still lowerabsolute pressures than those.

prevailing in tower I0. To this end, the residual oil is drawn from thebase of the tower I0 through line 42 by pump 43 and forced through theconvection and radiant tube banks 44, 45, of pipe still I3, where it isheated to the desired higher temperature and is discharged into tower at45, at which point a vaporlzing pressure of, say, 12 millimetersabsolute, is maintained. The pipe still heater is so designed that theoil passes through the heating tubes at high velocity (e. g. 6 to 8 feetper second) with extreme turbulence, whereby the danger of localoverheating and cracking of the oil is eliminated. A slight'pressure ismaintained at the discharge end of the outlet of the pipe still intotower I, by means of a valve 48|, in order to minimize vaporization ofoil in the tubes of the pipe still heater.

The lighter portions of the highly heated oil thus discharged into towerIl, including both gas oil and lubricating oil fractions, ash into vaporwhich ascends the column. Such portion as is not vaporized passesdownwardly as a liquid over stripping plates 41 and collects in theconstricted base 48 to a level 49 automatically controlled through theregulating action of float control 50 on the operation of are-circulating supply pump |00, to be referred to again hereinafter,which augments andmaintains constant the supply of hot liquid residueavailable for withdrawal through line 52 by pump 53.

In ascending tower l I, the oil vapors flashed of! therein undergofractionation by means generally similar to that already described fortower Ill. In addition to an entrainment separator 54, there areprovided two sets of i'racionating plates 55, 58, with cooperatingcollector plates 51 and 53, respectively, and reflux condensers 59 and30, respectively. This enables the lubricating oil components of theascending vapors to be fractionated, condensed and withdrawn from thetower in two cuts or fractions, one comprising relatively lightcomponents corresponding generally to light lubricating oil, and theother comprising medium and heavy lubricating oil. Division of thelubrieating oil condensate into this or any othe specific number offractions is a matter of choi e and depends largely upon considerationsof a practical nature unnecessary to discuss. Combining the twofractionating sections in a single tower, as here illustrated, iseconomical as to cost of construction and operation, but is notessential to the practice of the invention.

The vapors evolved at 46 pass upwardly through tower with a whirlingmovement, as

previously explained in connection with tower I0, in intimate contactwith-a moderately high-boiling lubricating oil reflux provided bycondensation in reflux condenser 59` of the heavier lubricating oilfractions of the vapors, this reflux being directed to the topniost ofthe plates 55 largely by side ring V`,59| and filming down over theremaining plates of this lower series. Lower boiling lubricating oilvapors pass beyond condenser59, whirling upwardly in contact withdescending iilxns of liquid lubricating oil reflux provided bycondensation in condenser 50 of those lower-boiling lubricatingconstituents which escape condensation therebelow. This reflux isdelivered to the topmost of plates 55 by circular distributor 60 Vaporsof gas oil and any other lighter products escape condensation in refluxcondenser 50 and pass on into water-cooled final condenser 8|, where theprincipal gas oil fraction is condensed, together with some malodorousand unstabilizing impurities. Residual uncondensed vapor and gases aredrawn away at high rate of speed through large diameter offtake 62 bysuitable exhausting means which, for this tower, may advantageously be a3-stage vacuum pump of the ejector type, conventionally indicated at602.

In a typical instance, the exhausting means just mentioned is operatedat such rate as to maintain an absolute pressure of about 4 millimetersof mercury at the top of the column just below the reflux condenser 60.Under such conditions, employing the special low resistance type offractionating plates hereinabove recommended, there is a vacuum drop orpressure increase of only about 8 millimeters down through bothfractionating sections and the entrainment separator 54, the absolutepressure at the discharge inlet 46 being thus only about 12 millimeters,while the pressure immediately below the collecting plates 56 ing pumps.

some malodorous and unstabilizing contaminantsy 61 and 58 -isapproximately 11 millimeters and 7 millimeters, respectively. 'I'heseilgures are of course not to be understood as in any sense restrictive,although they are characteristic of good practice within the scope ofthe invention.

Condensed light lubricating oil distillate is led away from collectingplate 58 through line 63 to constant level seal'tank 64, from which itis withdrawn by pump and pumped through line 66, heat exchanger 61,water-cooled cooler 68, and line 69 to storage. 4

Similarly, medium-heavy lubricating distillate condensed in the lowerfractionating section and collecting on plate 51 passes to storage byway of line 18, seal tank 1 I, pump 12, heat exchanger 13, water-cooledcooler 14 and line 15.

Pumps 65 and 12, like all other pumps in the system drawing liquid oilfrom a container maintained under high vacuum, are located a sufllcientdistance below their respective seal tanks 64 and 1| to ensure thenecessary head of oil on their A intakes for proper operation.

In a typical instance, the vapor temperatures in tower II may beapproximately` as follows: 560 F., just below collecting plate 51; 500F. just below reux condenser 59; 465 F. just below collecting plate 58;and 350 F. just below reflux condenser 68.

'Ihe reflux condensers 59 and 68 utilize as their cooling medium thecharging stock which passes through the cooling spaces thereof prior tobeing delivered into tower I8. For this purpose, the crude chargingstock line I1 is extended as shown on the drawing, the charging stockflowing therethrough and through the cooling spaces of the said refluxcondensers in the directions indicated by the arrows. The cooling of thelubricating distillates in heat exchangers 61 and 13 is similarlyeffected by means of the initial charging stock to be preheated, branchpiping 'I6 and 11, connected with charging stock line I1, being providedfor said heat exchangers 61 and 13, respectively, to attain this end.Suitable valves (not shown) may be provided in the piping system toregulate and vary the distribution of the crude charging stock to thecooling spaces of the several heat exchangers and reflux condensersreferred to.

The gas oil fraction condensed in condenser 6|, which amounts in atypical instance to around 12 'to 14 per cent of the crude chargingstock, is led away to storage by way of line 18, seal tank 19, pump 88and line 8|, the arrangement and automatic operation of pump 88 beingsimilar to that already described Afor the other similarly function-This gas oil fraction also includes as already stated.

The several seal tanks associated with tower are provided with suitablepressure balancing connections 64I, 1I I, 19|, as shown.

Unvaporized liquid oil which maybe at a temperature of around'600 F. ina typical instance, is drawn from the bas-e of tower II by pump 53 andforced at high velocity and with turbulent flow through tubebanks 82,83, of pipe still I4, being thereby heated to a still highertemperature,

l which is the maximum used and which may be from 700 to 725 F. At thistemperature, under slight pressure regulable by valve 83|, it is dislplace. AThe absolute pressure maintained at the flash vaporization-point84 in tower I2 should be charged into tower I2 at point '84, and flashvaporization of the heaviest lubricating oil fractions obtainable asoverhead distillates there takes the minimum for this location in any othe towers. A pressure of 6 millimeters absolute and vapor temperatureof about 670 F. at this point in tower I2 is good practice, with apressure of 3 millimeters and vapor temperature of 490 F. at the top ofthe column above the fractionatlng section comprising fractionatingplates 85, collecting ring 86 and entrainmentfseparator 86|. The heavylubricating oil vapors flashed oil at 84, together with a very smallpercentage of lower boiling vapors which are practically allnon-lubricants, pass upwardly through the system of fractionator plates(eight inrnumber in this instance) in intimate contact with down-flowingfilms of heavy lubricating oil vapors in reflux condenser 81 which maybe water-cooled as here shown. Distributor 81| delivers this liquid reuxto the topmost of plates 85.

Reflux condenser 81' is so operated, through ap.- propriate regulationof its cooling water, that a through large-diameter offtake 93 bysuitableA exhausting or vacuum-producing means, such as 3-stageejectorpump 682 aforesaid, the offtake connections of the columns and I2thereto being appropriately valved, e. g. as indicated at 62 I, 622 and623. The condensate formed in condenser 88, which commonly amounts toaround 0.5-per cent of the crude charging stock, comprises about equalproportions of lubricating oil and contaminants of malodorous andunstabilizing character. It is good practice to allow this smallquantityv of lubricating constituents to escape condensation incondenser 81 in order to ensure better separation of said contaminantsfrom the heavy lubricating distillate drawn off from plate 86. Thisheavy lube distillate passes l to storage by way of seal tank 84|, pump842,

water-cooled'cooler 843, and line 844; ,operation of the pump beingcontrolled by constant-level control float device 845. Seal tanks 84|and 98 are provided, respectively, with pressure-balancing connections846 and 98|, as shown.

The unvaporized liquid residue of the oil charged into tower I2constitutes the final residuum obtained from the primary distillationafter all the recoverable lubricating values have been vaporized fromthe initial charging material. This ilnal residuum, which may be in thenature of a flux oil, passes down over stripping plates 94 andaccumulates in the base of tower I2 to a level maintained constantthrough suitable connections of a float control device 95 to ux oil pump96 which draws ux oil, at a temperature of, say 650 F., from the base oftower I2 through line 95| at an automatically regulated rate anddelivers it to storage by way of line 91, heat exchanger 98, and line99. In heat exchanger 98, the extremely hot flux oil gives up a large.part of its heat units to the initial crude charging stock which hasalready been preheated to a considerable extent by passage throughreflux condensers and heat exchangers operated in conjunction with towerII, as already described.

Associated with pipe still charging pumps 43 and 53, which mayconveniently be electrically driven, are shown steam stand-by pumps 43|and 75 effected in various ways, but a good practical.

method is--that shown on the drawings wherein the portion of hot fluxoil (eig. at 650 F.) to be thus re-circulated is picked up by pump anddelivered through line IUI to sump 48 at the base of tower II, theoperation of the pump being automatically regulated as to rate by liquidlevel control device 50 suitably connected to the steam supply thereof,as shown. This very hot flux oil is thus combined with the somewhatcooler (e. g. 600 F.) unvaporized liquid residue resulting from theflash vaporization occurring in tower I I. The volume of the materialpumped through pipe still I4 by pump 53, and discharged at 84 into towerI2, is thus augmented materially and its total available heat contentmuch increased without the necessity of increasing the predetermineddesirable pipe still discharge temperature into tower l2.O Thisre-circulation of iiux oil from tower I2 therefore accomplishes thedesired purpose of effecting the maximum vaporization of heavy lubes intower I2 at a lower pipe still discharge temperature than wouldotherwise be feasible. An additional advantage gained by re-circulatingthe residual unvaporized material from the base of tower I2 is that itpermits maintenance of a constant thruput through the tubes of pipestill I4 and facilitates the operation at a constant dischargetemperature into tower I2, irrespective of possible variations inoperating conditions to a greater or less extent in the preceding partsof the distillation system.'

What is claimed is:v

1. In a plant for the manufacture of lubricating oil distillates,apparatus comprising a flash and fractionating tower, means fordelivering heated charging stock to the vaporizing zone of said tower,means for withdrawing fractionated products from said tower in sidestream, a second flash and fractionating tower, a pipe still heater,means for continuously withdrawing liquid oil residue from said firsttower, passing it ,through said heater and delivering it to thevaporizing zone of said second tower, means for withdrawing fractionatedproducts from said,

second tower in side stream, meansfor withdrawing liquid oil residuefrom said second tower and delivering it to the vaporizing zone of saidfirst tower, means for controlling the rate at which liquid oil residueis delivered to said first tower to maintain therein a body of liquidoil residue of substantially constant volume, and means for maintaininglow absolute pressure in said towers.

2. A high vacuum distillation system for the manufacture of minerallubricating oils as overhead distillates which comprises, incombination, a flash and fractionating tower provided with a vaporizingsection, an entrainment-separating section, a fractionating section andan accumulator for receiving unvaporized liquid oil residue, means fordelivering heated charging stock to said vaporizing section, means forwithdrawing vfractionated products from said tower in side withdrawingliquid oil residue from said accumulator, passing it through said heaterandv delivering it to the vaporizing section of said second tower, meansfor withdrawing fractionated products from said second tower in sidestream, means for conducting unvaporized liquid oil residue from saidsecond tower to said accumulator, means for varying the rate at whichliquid oil residue from said second tower is conducted to saidaccumulator to maintain therein a body of liquid oil residue ofsubstantially constant volume, and vacuum-producing means connected tothe vapor outlet of each tower for withdrawing vapors therefrom andmaintaining low absolute pressure therein.

3. A high vacuum distillation system for the manufacture of minerallubricating oils as overhead distillates which comprises, incombination, an initial iiash and fractionating tower provided with avaporizing section, an entrainmentseparating section and a fractionatingsection; means for delivering heated charging stock to said vaporizingsection; means for withdrawing fractionated products from said tower inside stream a second fiash and fractionating tower provided with avaporizing section, an entrainment-separating section, a fractionatingsection, a condensing means, and an accumulator for receivingunvaporized liquid oil residue; means "for withdrawing fractionatedproducts from said second tower in side stream; conduit means wherebycharging stock may be passed through said condensing means to serve ascooling medium therein and to be preheated prior to delivery to saidvaporizing section of said initial tower; a pipe still heater; conduitmeans whereby unvaporized liquid oil residue may be led from saidinitial tower, passed through said heater, and delivered to thevaporizing section of said second tower; a' third flash andfractionating tower provided with a vaporizing lsection, anentrainment-separating section and a fractionating section; a secondpipe still heater; means for continuously withdrawing liquid oil residuefrom said accumulator, passing it through said second heater anddelivering it to the vaporizing section of said third tower; means forwithdrawing fractionated products from said third tower in side stream;means for conducting unvaporized liquid oil residue from said thirdtower to said accumulator; means for varying the rate at which liquidoil residue from said third tower is conducted to said accumulator tomaintain therein a body of liquid oil residue of substantially constantvolume; and vacuum-producing means connected to the vapor outlet of eachtower for withdrawing vapors therefrom and maintaining low absolutepressure therein.

4. In a high-vacuum distillation system for the manufacture of minerallubricating oils as overhead distillates, the combination, with meansfor heating lubricating oil charging stock to flashing temperature, of atower having a vaporizing section into which the heating means isarranged to discharge heated charging stock; an entrainment separatingsection above said vaporizing section, comprising means for laterallydeflecting and imparting considerable velocity and centrifugal action tothe ascending vapors to throw out entrained liquid, and means forcollecting resultant separated liquid; a fractionating section abovesaid entrainment-separating section comprising a series of alternatingring plates and deflector plates and curved guide'vanes associated withsaid plates to impart whirling l movement to the ascending vapors; meansdis-l posed above said fractionating section for condensing vaporstherefrom and for delivering liquid reflux condensate "thereto: a secondfraction,

ating section disposed above said condensing means and comprising aseries oi alternating ring plates and deilector plates and curved guidevanes, associated with said plates to impart whirling movement to theascending vapors; means disposed above `said second fractionatingsection for condensing vapors therefrom and for deliver-

